Gmoshkin/fix http server panics

Summary

• chore: remove redudnant instance_count calculations from get_replicasets_info

• chore: cleanup http_server somewhat

• fix: remove the potential for panics in http_server.rs

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• Close #564 (closed)
• Follow-up for !918 (merged)
• Target branch: master
• Changes should be cherry-picked to 24.2: yes